1. Field of the Invention
The present invention relates to a fixed-type or a sliding-type constant velocity universal joint which is employed in a power transmission system of an automobile or various industrial machines and is incorporated in a drive shaft or a propeller shaft employed in, for example, a 4WD vehicle or an FR vehicle. The invention also relates to an inner member constituting part of the constant velocity universal joint.
2. Description of the Related Art
A fixed-type constant velocity universal joint (a Rzeppa type constant velocity universal joint: BJ) has been employed as a connection joint for a drive shaft or the like of an automobile. For example, such a universal joint comprises: an outer ring serving as an outer member and having curved track grooves formed axially in its spherical inner diameter surface; an inner ring serving as an inner member and having curved track grooves formed axially in its spherical outside diameter surface; a plurality of torque transmission balls arranged in respective ball tracks each constituted by one of the track grooves of the outer ring in combination with the corresponding track groove of the inner ring; and a retainer having pockets for holding these balls. The plurality of balls are retained in the respective pockets formed in the retainer and are arranged at regular intervals in the circumferential direction.
When this constant velocity universal joint is employed in a drive shaft, a shaft portion (a driven shaft) integrally extending from one end of the outer ring in the axial direction thereof is connected to a wheel-bearing apparatus, and a shaft (a driving shaft) spline-fitted into a shaft hole of the inner ring is connected to a sliding-type constant velocity universal joint. Even when the outer ring and the inner ring are angularly displaced between the respective two axes of the shaft portion of the outer ring and the shaft in the inner ring, each of the balls retained in the pockets of the retainer is always held in a plane bisecting an operational angle at any operational angle. Therefore, the constant velocity of the joint is ensured. Here, the operational angle refers to an angle formed by the shaft portion of the outer ring and the shaft of the inner ring.
As for the inner ring of a constant velocity universal joint, in order to increase the strength, a hardened layer is formed through heat treatment to extend the life time of the inner ring, whereby the product lifetime of the constant velocity universal joint is extended (see, for example, Japanese Patent Laid-Open Publication No. 2000-227123).
Generally, the inner ring of the abovementioned constant velocity universal joint is formed of a steel material capable of being hardened by quenching. Furthermore, in this inner ring, a hardened layer 121 by means of induction hardening is formed on an outside diameter surface 104 contacting the inner diameter surface of the retainer and on track grooves 105 on which a high contact pressure is exerted due to the rolling motion of the ball, as shown in FIGS. 8 and 9.
Here, FIG. 8 is a cross-sectional view taken along is the line F-F in FIG. 9, and FIG. 9 is a cross-sectional view taken along the line E-O-E in FIG. 8. In FIG. 9, hatching for indicating the cross-sections is omitted, and a portion in which the hardened layer 121 is formed is hatched.
A female spline portion 123 is formed axially in the inner diameter surface of a shaft hole 122 of an inner ring 106, and a male spline portion is formed axially in the outside diameter surface of a shaft (not shown). By inserting the shaft into the shaft hole 122 of the inner ring 106, the male spline portion of the shaft is brought into engagement with the female spline portion 123 of the inner ring 106, whereby the shaft is fixedly connected to the inner ring 106 such that torque can be transmitted therebetween.
Generally, a hardened layer by means of heat treatment is not formed in the female spline portion 123 of the shaft hole 122 of the inner ring 106 into which the shaft is spline-fitted. This is because when a deep hardened layer by means of induction quenching is formed in the female spline portion 123 of the inner ring 106, ensuring the accuracy of the spline fit becomes difficult since deformation occurs due to the heat treatment.
However, when a comparison is made between the inner ring 106 in which the hardened layer by means of induction quenching is not formed in the female spline portion 123 and an inner ring in which the hardened layer by means of induction quenching is formed in the female spline portion 123 thereof, the strength of the inner ring 106 in which the hardened layer by means of induction quenching is not formed becomes lower than the other.
As discussed above, if the hardened layer by means of induction quenching is formed in the female spline portion 123 of the shaft hole 122 in the inner ring 106, the strength of the inner ring 106 can be ensured. However, in this case, difficulty lies in ensuring the accuracy of the female spline portion 123 since deformation occurs due to the heat treatment. On the other hand, if the hardened layer by means of induction quenching is not formed in the female spline portion 123 of the shaft hole 122 in the inner ring 106, the accuracy of the female spline portion 123 can be ensured, but difficulty lies in enhancing the strength of the inner ring 106.
As described above, depending on whether or not the female spline portion 123 of the shaft hole 122 of the inner ring 106 is subjected to the heat treatment, there are advantages and disadvantages in terms of the strength of the inner ring 106 and the accuracy of the female spline portion 123. However, there is no effective means for providing strength and accuracy at the same time.
Furthermore, in modern automobiles, there is a demand for a drive system having reduced backlash in order to improve the riding comfort of the automobiles. Generally, for the spline fit between the inner ring 106 and the shaft, a twist angle is additionally provided in the male spline potion of the shaft to thereby reduce the amount of backlash of the spline fit between the inner ring 106 and the shaft. However, since the twist angle is asymmetric in directions of twist (two directions corresponding to the forward and backward motions of an automobile), the strength of the shaft varies depending on the load-applied direction of twist torque.
In order to prevent this, it has been proposed that, in a pair of left and right drive shafts employed in an automobile, the direction of twist angle for male spline portions of the shafts is set separately for each of the left and right drive shafts (see, for example, Japanese Utility Model Publication (kokoku) No. Hei. 6-33220). However, if such means is employed, although the same left and right shafts can be employed for production, only the male spline portions have different shapes for the left and right sides, resulting in an increase in the kinds of products. In addition, since it is difficult to visually distinguish the difference in the direction of the twist angle, product management becomes difficult. Moreover, since separate production lines must be provided for the left and right portions, a problem arises in that productivity is lowered.
In order to solve this problem, it has been proposed that the backlash of the spline fit between an inner ring 106 and a shaft is eliminated by providing crowning having a left-right symmetric shape in the male spline portion of the shaft to thereby eliminate the dependency of the strength on the load-applied direction of twist torque (see, for example, Japanese Patent Laid-Open Publications Nos. 2001-343023, 2001-287122, and 2001-323920). However, these means are employed for a shaft as in Japanese Utility Model Publication (kokoku) No. Hei. 6-33220 mentioned above. Therefore, there is no means for solving the problems for the female spline portion 123 of the inner ring 106.